As precision devices such as disk drives are reduced in size, the components of the disk drive assembly likewise must be reduced in size. Whenever the ball bearings of the disk drive motor are made smaller to fit into the small form factor disk drives, the ability of the ball bearing assemblies to withstand impact and shock forces is significantly reduced. Although the design of the component, specifically the ball bearing, may be substantially the same as the design for its larger predecessor counterpart, the reduced load rating of the individual components of the ball bearing assembly cannot accommodate the same level of forces without experiencing failure as may be exerted onto the larger ball bearing assemblies. Because the precision manufacture of small form factor disk drives requires extremely fine tolerances and provides little allowance for deviations in performance, augmentation of the shock tolerance of the reduced size components is required in order that these disk drives be sufficiently rugged to withstand impacts and shocks which can be commonplace.
Shocks exerted onto the ball bearings can be axial, in the direction of the axis of the shaft on which the bearing is disposed; radial, substantially perpendicular to the axis of the shaft; or a combination of radial and axial should the impact force be exerted at a distance displaced from the plane of the ball bearing. In addition to the force factors of axial and radial force vectors, a moment or torquing about the ball bearing assembly can occur due to an off-line force coupled with one of the ball bearing assemblies acting as a pivot and the other ball bearing assembly being the source of the resistance to the moment. For ball bearings which absorb a substantially axial impact, the load of the impact is shared by all of the balls in the ball bearing assembly. However, if a ball bearing assembly is impacted in a radial direction, quite possibly only one of the balls of the ball bearing assembly may carry the entire load of the impact.
As the impact occurs in the radial direction, the force exerted on the outer race will impart a force to one or more of the balls placed between the outer race and the inner race, and then only on those balls which are resident in the semicircular annular region between the outer and inner races and on the side of the shaft where the impact force is focused.
As will be appreciated whenever there are a small number of balls in the ball bearing assembly, six for example, and one of those balls is directly between the axis of the force impacting the outer race against the ball and toward the axis of the shaft, all or substantially all of the impact force will be carried by that single ball. As the force is transmitted against the inner race by the ball or balls positioned to carry the impact force, the shaft of the device, such as a disk drive for example, will resist any movement of the inner race. Depending on the magnitude of the shock or impact force, the material in the inner and outer races or the balls will begin to deform to a varying extent. Responding to a small force, the ball will tend to penetrate the respective race surfaces by a corresponding small amount. No damage will occur so long as the penetration of the ball into the race of the ball bearing assembly or the ball deformation is totally within the elastic limits of the material from which the inner and outer races are fabricated. Once the impact force has been dissipated and the ball bearing assembly returns to its unaffected state, the elastic nature of the deformation will permit the balls and races of the ball bearing to restore themselves to an unaffected shape and surface.
On the other hand, should the shock or impact force be sufficient to deform the inner and outer races or balls beyond their elastic limit and into a region of plastic deformation, the surface of the race or ball is deformed to the point where the race or ball surface will not resume its original configuration and thus will present an irregularity as the balls roll past the point of penetration causing vibration and runout of the bearing. Due to bearing vibration and runout, there is a resulting runout of the rotor of the disk drive motor. Since the rotor is attached to or is part of the hub which in turn supports the magnetic disks, there will be a correspondent runout of the disk. Runout of the disk is unacceptable beyond very small limits because the runout will cause recording track misregistration and it will be impossible to read data which has been previously recorded on the magnetic disk surface in a circular track of a precise radius.
Similarly, if data is being recorded on a disk which is experiencing runout beyond an acceptable limit, the recording track being laid down on the surface of the disk may not correspond to the position of the track at a future time as an attempt is made to read the data from the magnetic disk.
While the deformation in the inner and outer race surfaces may be only measurable in micro inches in a very small precision ball bearing, these small surface disruptions are sufficient to severely degrade the performance of the bearing and the device incorporating the bearing.
Due to manufacturing tolerances, design constraints as well as the inherent elasticity of the materials from which the bearings, shaft and the hub of the disk drive are manufactured, some very limited movement of the rotor may occur about and/or along an X or Y axis and along the Z axis, where the axis of rotation and the Z axis are coincident and the X and Y axis are radial to the shaft. Depending upon the point of impact or shock, the most likely motion will be a rotation of the rotor may occur about either or both of the X and Y axes. This rotation of the hub and consequently the inner and outer races will compress at least one ball in each of the ball bearing assemblies. Depending on the amount of the rotational movement and the spacing of the bearings, the compression may exceed the elastic deformation limit of the material of the races.
Where the bearing assemblies are both placed at positions near one end of the rotor and consequently one end of the shaft, the potential for bearing damage is increased. This increased potential for damage is due to the lever arm through which the impact force may act, thereby increasing the force on the balls and races. The only viable approach to preventing any damage caused by shock or impact forces is to cushion the device to absorb some or all of the forces or to block transmission of the forces to the bearing assemblies.